(a). Field of the Invention
The present invention relates to a waveguide type semiconductor photodetector and, more particularly, to a semiconductor photodetector comprising a waveguide having a function of power attenuation of incident light to a suitable level based on the amplitude thereof to obtain excellent output characteristics against signal distortion in a wide frequency range. Such a photodetector device is suitable for a communication system using a AM-FDM (amplitude modulation/frequency division modulation) technique.
(b). Description of the Related Art
A waveguide type photodetector device is desired in the subscriber optical modules in an optical communication system, which has a function for limiting the maximum amplitude of the output photo-current at a predetermined level irrespective the amplitude of the incident light received in the photodetector device. The applicant has proposed, in Patent application Ser. No. 7-243725 (published on Mar. 7, 1997 as Patent Publication No. JP-A-9(1997)-64399), a waveguide type semiconductor photodetector device having a variable photo-sensitivity. In the proposed device, a multiple quantum well (hereinafter called MQW) structure constituting an optical attenuation section is disposed at the input of the photosensitive element to utilize a quantum confined stark effect (hereinafter called QCSE), which is generated upon applying a source voltage thereto.
In general, it is known that an optical absorption spectrum in the MQW structure as well as the signal change therein upon application of a source voltage varies depending on the polarization of the incident light. Therefore, the amount of optical attenuation in the MQW structure varies depending on the polarization of the incident light also in the proposed variable-sensitivity photodetector device, thereby exhibiting a polarization dependency of the photosensitivity as a photodetector device. Accordingly, it is difficult to limit the output level of the photosensitive element in the proposed device since the polarization of a signal light received by an optical receiver generally varies at random in an optical communication system.